Regeneration of photographic silver bleach solutions



REGENERATION 9F PHOTOGRAPHIC SHJVER BLEACH SOLUTIONS Lloyd E. West,Rochester, and Bernard A. Hutchins,

Livonia, N.Y., assignors to Eastman Kodak Company,

Rochester, N.Y., a corporation of New Jersey .NoDrawing. Filed Apr. 13,1955, Ser. No. 501,188 '3 Claims. (Cl. 96--60) This invention relates tothe regeneration of spent photographio silver bleach solutions withwater-soluble persulfates.

In the processing of color film and in many black and white photographicprocesses the silver which forms upon developing is converted into ahypo-soluble silver salt which is subsequently removed by fixing. Acommonly employed silver bleaching solution consists essentially of anaqueous solution of an alkali ferricyanide and an alkali bromide withpossibly other chemicals added. The use of these solutions for bleachingdeveloped photographic emulsions depletes both the ferricyanide and thehalide ion therein. The efiiciency of the bleaching solution diminishesupon continued use and, therefore, regeneration thereof would bedesirable after each use.

The prior art has suggested regeneration of bleach solutions with alkalimetal bromates but the expense involved in that method makes it somewhatquestionable whether or not regeneration offers anything over bleachsolutions newly prepared. Electrolytic rejuvenation methods have alsobeen disclosed but there are objections thereto such as the high capitalinvestment necessary, high maintenance costs and the formation of sludgein bleach solutions treated in this manner.

One object of our invention is to provide a method for regeneratingexhausted spent silver bleach solutions, characterized by the ease ofoperation and the practical nature thereof. Another object of ourinvention is to provide a method for regenerating spent bleach solutionsin which the products incorporated in the solution as a result of theregeneration do not exhibit any elfects deleterious to its value inprocessing photographic products. A further object of our invention isto provide a method for regenerating bleach solutions which imparts thedesired efiect thereto rapidly and under mild conditions. Other objectsof our invention will appear herein.

We have found that spent ferricyanide type silver bleach solutions maybe regenerated in a time on the order of 30 minutes at usualtemperatures by the use of the water soluble persulfates over a wide pHrange such as pH 3-10. We have found that the presence of persulfates,even in excess, does not impair the bleaching action of ferricyanidetype bleaching solutions and is not harmful to the photographic materialprocessed therewith. We have found that spent ferricyanide type bleachsolutions may be rapidly rejuvenated in this manner, with only a buildup of sulfate ion therein which, when desired, can be readily removed bythe addition of calcium chloride without any undue loss in concentrationof the active constituent therein, except possibly as the voluminousprecipitate formed may entrap a certain amount of bleaching reagents.

Our invention involves the use as the regenerating agent of a watersoluble persulfate such as of ammonium,

sodium or potassium in an amount at least chemically equivalent to theferrocyanide formed upon use of the bleaching solution. The addition ofan excess of the persulfate to the solution not only does not interferecyanide present particularly where pH control is im-' portant.

Because the conversion of silver to silver bromide in the bleachingoperation reduces the bromide content of the bleaching solution, it isusually desirable to also add sodium bromide, to the spent bleachingsolution in sulficient amount to bring the bromide content thereof up tothat desired. Sometimes it may be desirable to supply additionalferrocyanide to the spent bleaching bath to restore the ferricyanidecontent'to approximately that originally present. In a typicalregeneration procedure the content of potassium ferrocyanide isdiminished by conversion to ferricyanide after the addition of potassiumpersulfate to the spent bleaching solution as illustrated in thefollowing table:

rejuvenated by our procedure in which a water soluble persulfate isadded up to the point where the sulfate salt which results therefromreaches its saturation equilibrium in the solution. This is not apt tobe reached under actual conditions. However, if this point is reachedthe sulfate ion content may be removed by the addition of calciumchloride or calcium bromide thereby precipitating calcium sulfatewithout seriously altering the concentrations of the active constituentexcept as the precipitate carries down with it some of the ferrocyanideor ferricyanide which is present in the solution. We have found thatpotassium persulfate is probably more convenient to use as theregenerating agent primarily because of its low cost and availability.In the case of ammonium persulfate although the pH of the bleachsolution is decreased the decomposition product thereof is more solubleand, therefore, ammonium persulfate is quite useful in the repeatedrejuvenation of the bleach bath. The persulfate added to the spentbleach solution quantitatively oxidizes any ferrocyanide therein toferricyanide and with the addition of further alkali metal bromide andadditional ferrocyanide, if desired, the bath may be brought to acomposition which is substantially as effective for bleaching purposesas the original bleaching solution.

The following example illustrates the regeneration of a spent bleachingsolution in accordance with our invention:

A photographic bleach solution having the following formula: Na Fe(CN).10H- O, 1 gram; Na Fe('CN) 23 grams; NaBr, 7.5 grams; water to make 1liter, which bath had a pH of 8.3 was employed to bleach a quantity ofcolor developed print material. The spent bleaching solution whichresulted had the following composition per liter; Na Fe(CN) .l0H O, 9.2grams; Na Fe(CN) 12.9 grams; NaBr, 4.75 grams. By calculation it becameapparent that a of the following quantities Patented July 12, I960 g f.t 2,944,895

would be needed to restore the spent bleaching solutio to its originalstrength per liter of solution:

Na Fe (CN) IOH O 8. 9-grams; K S 4.84 grams;-NaBr, 2.75 grams. The

until solution was complete. The mass was allowed tostand for 1 or 2hours and where desirable, the solution was filtered. The regeneratedbleach was then analyzed for ferrocyanide and ferricyanide content andif the pH value was critical that was determined also. When theferricyanide content was below that required, amounts of ferrocyanideandpersulfate were additionally added to bring the concentration to thedesired point; Experience indicates that it is desirable to use'a slightexcess of the persulfate and the ferrocyanide to obtain the desiredferricyanide concentration because of the increase in volume uponaddition of the chemicals. When the pH of the bleach is critical thisvalue may be adjusted by the addition of an appropriate chemical such assodium hydroxide, borax or boric acid.- After the spent bleach solutionhad been regenerated to bring the contents of ferricyanide and sodiumbromide to approximately those of the original solution, furtherdeveloped print material was processed therewith. It was found that thebleaching efiectiveness of the regenerated solution was substantiallyequivalent to that of the original bleach solution.

Our invention is also useful in preparing fresh bleach initiallystarting with ferrocyanide and persulf ate together with the additionof'alkali metal bromide thereto.

It is to be understood that we do: not wish" to be limited to thespecific embodiment described herein except as defined. by the appendedclaims We claim:

1. The processing of developed silver halide photographic emulsionlayers which comprises treating said layers with a bleach solutioncontaining analkali metal ferricyanide and an alkali metal bromide,adding'to the spent bleaching solution which results a water-solublepersulfate in an amount approximately chemically equivalent to theferrocyanide present in the spent liquor plus sodium bromide insuflicient amount to restore the bromide content of the bleach solutionand bleaching further 7 developed silver halide photographic emulsionlayers with the regenerated bleach solution thus obtained.

2. The processing of developed silver halide photographic emulsionlayers which comprises treating said layers with a bleach solutioncontaining an alkali metal ferricyanide and an alkali metal bromide,whereby a spent bleaching solution is obtained in which ferricyanide isreduced to ferroc'yanide, followed by adding to the spent bleachingsolution a water-soluble persul'fate in amount approximately chemicallyequivalent to the ferrocyanide therein and an alkali metal bromide insufficient amount to substantially restore the original bromide contentof the bleached solution.

3'. The processing of developed: .silver halide photographic emulsionlayers which comprises treating said layers with a bleach solutioncontaining sodium ferricyanide and sodium bromide, whereby theferricyanide of the solution is reduced to ferrocyanide, followed byadding to the spent bleaching solution potassium persulfate in an amountapproximately chemically. equivalent to the ferrocyanide present in thespent solution and sodium bromide in a sufliicient amount tosubstantially restore the original bromide content of the bleachedsolution.

Theoretical Chemistry, vol. 10, page 467.

General Chemistry, Pauling, 1947, pages 184-185.

Disclaimer 2,944,895.LZog 0Z E. W est, Rochester and Bernard A.Hutchins, Livonia, N.Y. REGENERATION OF PHOTOGRAPHIC SILVER BLEACHSOLUTIONS. Patent dated July 12, 1960. Disclaimer filed Aug. 26, 1974,by the assignee, Eastman Kodak Company.

Hereby disclaims the entire remaining portion of the term of saidpatent.

[Oyficial Gazette April 29, 1975.]

1. THE PROCESSING OF DEVELOPED SILVER HALIDE PHOTOGRAPHIC EMULSIONLAYERS WHICH COMPRISES TREATING SAID LAYERS WITH A BLEACH SOLUTIONCONTAINING AN ALKALI METAL FERRICYANIDE AND AN ALKALI METAL BROMIDE,ADDING TO THE SPENT BLEACHING SOLUTION WHICH RESULTS A WATER-SOLUBLEPERSULFATE IN AN AMOUNT APPROXIMATELY CHEMICALLY EQUIVALENT TO THEFERROCYANIDE PRESENT IN THE SPENT LIQUOR PLUS SODIUM BROMIDE INSUFFICIENT AMOUNT TO RESTORE THE BROMIDE CONTENT OF THE BLEACH SOLUTIONAND BLEACHING FURTHER DEVELOPED SILVER HALIDE PHOTOGRAPHIC EMULSIONLAYERS WITH THE REGENERATED BLEACH SOLUTION THUS OBTAINED.